This invention relates to urea-formaldehyde resins modified with melamine. Upon curing, these resins are characterized by low formaldehyde emission. The invention also relates to processes for preparing these resins, and to particleboard and other composites made using these resins as binders.
Urea-formaldehyde resins are widely used in industry as adhesives, particularly as binders for particleboard and other composites made from small pieces of wood. Urea-formaldehyde (UF) resins have the advantages of low costs, rapid cure, processing convenience, and clear color. As employed in the manufacture of composite board products, very short press cycles can be achieved with urea-formaldehyde adhesives. By adding a catalyst to a UF resin, the rate of cure can be adjusted to essentially any desired speed.
One disadvantage of the UF resins is the emission of formaldehyde fumes, both during production processes and, over a period of time, from the finished products. Particleboard and other composite panels made with UF resin binders contain cured resin films that have very large surface areas, which appear to enhance formaldehyde release.
The causes for formaldehyde release are complex. From the curable resin composition itself, free unreacted formaldehyde may evolve. When a resin is used as the binder for a composite panel, the formaldehyde may dissolve in moisture in the wood pieces, and its vapor pressure and its release rate may change with changes in air humidity and in product humidity. In particleboard, formaldehyde emissions can come from formaldehyde which was bound to wood cellulose during the hot press cycle, but which may slowly hydrolyze under the influence of acidic humidity in the wood. It may also result from the degradation of incompletely cured resin or resin components such as methylolurea.
Conventional cured UF resins may contain methylol, methylene ether, and other reaction products which possibly hydrolyze back to formaldehyde. The weakest links are in the cellulose-resin linkage, the hemiacetals, ethers, and methylols. The oxygen-free methylene linkage is the most resistant to hydrolysis, and, after assuming a hydrolysis mechanism, recent attempts to limit or eliminate formaldehyde release have sought to increase methylene linkages as compared with other group linkages in the resin.
Other paths have been explored over the last few years for reducing formaldehyde release. These include coating applications, chemical treatments before or after resin application, the use of resin additives, and new resin formulations. In such new resin formulations, the mole ratio of formaldehyde to urea has been slowly decreased over the years, but reductions in this ratio generally have tended to weaken the internal bond in particleboard, even though reducing residual formaldehyde and formaldehyde emission.
The copending patent applications of James H. Williams, Ser. Nos. 416,573 and 416,574, both filed Sept. 10, 1982, now U.S. Pat. No. 4,410,685, issued Oct. 18, 1983, and U.S. Pat. No. 4,409,293, issued Oct. 11, 1983, respectively, are concerned respectively with a new UF resin formulation that is hydrolytically stable and with particleboard made from this new resin. The resin itself is a UF liquid base resin having a UF mole ratio of substantially 1:1. This resin contains essentially no free formaldehyde. When cured, it contains substantially more methylene groups than methylene ether groups. These applications also disclose processes for preparing the resin, and processes for using it as a particleboard binder. Both of these applications are expressly incorporated herein by reference.